diff --git a/trigger/categories/lib.php b/trigger/categories/lib.php
index 97ed716f..9d0ca44c 100644
--- a/trigger/categories/lib.php
+++ b/trigger/categories/lib.php
@@ -76,6 +76,9 @@ public function get_course_recordset_where($triggerid) {
         $allcategories = [];
         foreach ($categories as $category) {
             array_push($allcategories, $category);
+            if (!isset($categoryobjects[$category])) {
+                continue;
+            }
             $children = $categoryobjects[$category]->get_all_children_ids();
             $allcategories = array_merge($allcategories, $children);
         }